Aftertreatment systems in current direct-injection natural gas engines are required to meet emission levels of the most stringent regulations. Due to the nature of the combustion process across the engine operating range, emission levels of particulate matter (PM), nitrogen oxides (NOx) and unburned hydrocarbons (UHC) are above regulated levels unless aftertreatment systems are employed. These aftertreatment systems can include particulate filters, oxidation catalysts and reduction catalysts. It is preferable to reduce the dependency on these aftertreatment systems to meet emission regulations, and ideally to eliminate their requirement, by improving the in-cylinder combustion process of the engine.
Various techniques have been studied and employed by the industry to improve the “quality” of in-cylinder combustion by enhancing the mixing between fuel and a gas comprising oxygen, such as air. As used herein the term ‘air’ is used interchangeably with the phrase ‘gas comprising oxygen’. It is understood in its broadest sense the gas comprising oxygen can be gases other than or in addition to air. Mixing techniques widely employed include using higher injection pressure, optimizing charge air motion such as swirl and tumble, optimizing injector nozzle geometry and combustion chamber geometry, and using multiple pulses for fuel injection.
Increasing fuel injection pressure is one of the most effective methods for improving fuel-air mixing for diesel engines. However its application in natural gas engines has some severe obstacles. First, a gaseous fuel system with higher pressure rating requires special design and material that significantly increases the fuel system cost while reducing the reliability or lifetime of the components. Secondly, higher injection pressure that is suitable for high load conditions may not be suitable for lower load conditions (an issue with turn down ratio), and thus this puts more pressure on injector/fuel system design to cover a wide range of operating pressures for different load conditions. Finally, the mixing process of gaseous fuel is somewhat different from that for a liquid fuel. In particular, gaseous fuel does not have the atomization phase which entrains air to the center of the fuel jets. As a result, the effect of higher injection pressure on improving fuel/air mixing during the spray breakup and atomization process does not apply to gaseous fuel jets.
Improving mixing through charge air motion is effective for certain engine modes but not for others. Since little control can be applied to change the charge air motion once the engine design is fixed, optimization at different engine operating points is not feasible. As an additional drawback, since charge air motion mainly affects the bulk motion of the charge, its effectiveness on local mixing is limited.
Similar to charge air motion, injection nozzle and combustion chamber geometry has a different impact on the combustion process under different operating conditions. While optimization can be achieved under certain engine operating points, a global optimization over the engine map is difficult.
Multiple injection pulses have been proven to be effective in diesel engines for PM reduction. Its application is, however, limited by the response time of the injector (controller and driver), total injection duration and injector dynamics. Its effectiveness on gaseous fuelled engines has not yet been proven.
An important issue none of the above techniques is able to fully address is transient behavior of the engine. In a transient state, the engine can shift from low load to high load operation in a short duration. The amount of fuel injected per cycle increases rapidly to provide increased torque that matches the requirement of increased load. The air handling system usually lags the fuel system during transient. As a result, the engine experiences temporary “starvation” of oxidant, or in technique term, an increase of equivalence ratio which leads to elevated levels of PM and carbon monoxide (CO) in the exhaust.
Certain air-assisted liquid fuel injection techniques are known. Pressurized air is employed as a high pressure source for injecting liquid fuel during the injection event. The pressure of air defines the injection pressure, and air supply cannot be turned off since it is required for fuel injection. The liquid fuel is mixed with compressed air in a fuel injector, since it is not practical to mix liquid fuel and air further upstream from the injector. The liquid fuel and air do not form a homogenous mixture prior to injection. In air-assisted injection, air is used as a driver to drive the liquid fuel into the cylinder and help atomization of the fuel. There is effectively no “mixing” between air and the liquid fuel prior to injection.
The state of the art is lacking in techniques for improving combustion quality of gaseous fuelled, direct-injection internal combustion engines throughout the engine map and during transient conditions to decrease emission levels whereby dependency on aftertreatment systems is reduced and ideally eliminated. The present method and apparatus provide a technique for improving the quality of the combustion process in a gaseous fuelled, direct-injection internal combustion engine.